1/27/2010
Physics 110G
Energy and Power
These are the basic definitions you will need.
Energy: A measurement of physical change
or the possibility of physical change (potential energy)
Work: That which causes change
Power: A measure of the amount of physical change in a
specific amount of time
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Physics 110G
Energy and Power
These are the common units of work and energy.
calorie: the amount of energy necessary to heat 1 gram of water
by 1 C
Calorie (kilocalorie) : 1000 calories
joule: the amount of energy necessary to raise 1 kg by 10 cm
kilojoule: 1000 joules
megajoule: 1000 kilojoules
kilowatt-hour (kWh): 861 Calories
British thermal unit (BTU): 1055 joules
2
Quad: 1015 BTU
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1/27/2010
Physics 110G
Energy and Power
A chocolate chip cookie vs. dynamite
A single gram of chocolate chip cookie contains
5 Calories.
Compare this to a gram of TNT which has 0.65
Calories
THE COOKIE HAS 8 TIMES AS MUCH ENERGY!!!
So why does it not cause more damage?!!!
The answer:
An explosion is caused by releasing energy over
a short period of time.
This is called power!
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Physics 110G
Energy and Power
These are the common units of power.
watt: 1 Joule per second
100 watts : bright light bulb
1 hp: approximately 1 watt
kilowatt: 1000 watts
megawatt: 1000 kilowatts
gigawatt: 1000 megawatts
terawatt: 1000 gigawatts
What appliance in your home uses the most power?
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2
1/27/2010
Physics 110G
Energy and Power
Chevy Volt
http://www.chevrolet.com/electriccar/
40 miles per charge (16 kWh ) -> 16 kWh each way per drive to and from El Paso
Estimated Price in 2011 -> $35,000
Keep for 10 years
Electrical Costs: 1000 kWh = $120.00
Toyota Corolla
37 mpg -> 1 gallon per drive to and from El Paso
Estimated Price in 2011 -> $22,000
Keep for 10 years
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Gasoline Costs: 1 gallon = $3.00
Physics 110G
Energy and Power
Price for one-way trip of 40 miles
Total Cost =
Chevy Volt
Energy Cost =
Vehicle Cost =
$120.00 16 kWh
$1.86

1030 kWh 40 miles 40 miles
$1.86
$4.79


40 miles 40 miles
$6.65
40 miles
1 day
$35, 000 1 year
$4.79
2

10 years 365 days 40 miles 40 miles
Toyota Corolla
$3.00 1 gal
$3.00

1 gal 40 miles 40 miles
1 day
$22, 000 1 year
$3.01
2
Vehicle Cost =

10 years 365 days 40 miles 40 miles
Energy Cost =
6
Total Cost =
$3.00
$3.01


40 miles 40 miles
$6.01
40 miles
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1/27/2010
Physics 110G
Energy and Power
How to Measure the Impossible
(And now for something completely different!)
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Physics 110G
Energy and Power
How can we measure the size of the earth?
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1/27/2010
Physics 110G
Energy and Power
How can we measure the size of the earth?
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Physics 110G
Energy and Power
How can we measure the size of the earth?
Eratosthenes 235 BC
in Alexandria
In Syrene (which is 800 km away)
the angle is 0˚
7.2˚
7.2
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1


360 350 50
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Circumference  50  800 km  40000 km
Circumference 40000 km
Radius 

 6366 km
2
2
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1/27/2010
Physics 110G
Energy and Power
How can we measure the size of the moon?
radius of the moon 
radius of the earth 6366 km

 1819 km
3.5
3.5
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Physics 110G
Energy and Power
How can we measure the distance to the moon?
12distance the moon  coin distance radius of the moon  220 1819 km  400,180 km
coin radius
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1/27/2010
Physics 110G
Energy and Power
How can we measure the distance to the sun?
90˚
0.15˚
89.85˚
distance to the sun  400  distance to the moon  400  400,180 km  160,072,000 km
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Physics 110G
Energy and Power
How can we measure the size of the sun?
sun radius 
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moon radius
distance to the sun 
moon distance
1819 km
160, 072, 000 km  727, 600 km
400,180 km
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1/27/2010
Physics 110G
Energy and Power
So how well did we do?
Our
Measurement
Actual
Value
Percent
Difference
Earth radius
6366 km
6378 km
0.2%
Moon radius
1819 km
1738 km
4.7%
Moon Distance
400,180 km
384,000 km
4.2%
Sun Distance
160,072,000 km
149,600,000 km
7%
Sun Radius
727,600 km
695,000 km
4.7%
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Physics 110G
Energy and Power
Let’s try this ourselves.
Volunteer 1: Measure the diameter of this circle (the length of the line in the
drawing). Do not tell anyone but write down the answer and put it in the envelope
provided. (All measurements should be in inches.)
Volunteer 2: Close one eye and hold a red chip out away from your eye until it just
covers the circle.
Volunteer 3: Measure the distance from volunteer
two’s eye to the chip. Be as accurate as
possible. Write the answer on whiteboard.
Volunteer 4: Measure the distance from
volunteer 2 to the screen. Write the answer on
the whiteboard.
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Dr. Mike (and Volunteer 5) will now attempt to guess the answer in the envelope
using the equations for the distance to the moon.
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